The present invention relates to spread spectrum communication and more particularly, to spread spectrum communication techniques and applications using signature sequences.
Spread spectrum communication techniques are used for information carrying signals in a variety of communication systems because of their ability to reduce the effects of certain transmission impairments. Many multi-user communication techniques suffer co-channel interference, multiple access interference and intersymbol interference. The use of spread spectrum transmission and reception attenuates these interference types.
In Local Area Networks (LANs) there is an increasing desire to extend or replace wired LANs in order to increase functionality and maximise the number of potential applications for such systems. This trend facilitates the growing need for wireless access. This wireless access allows mobile computer users to remain in contact with a given corporate LAN over short distances. Currently available systems provide such connections using either radio or infrared communication technology. For certain system requirements, this communication is adequate. However, the application of such technologies to wireless LANs is relatively new and therefore can prove to be both expensive and unreliable. Furthermore, the data transmission rates achievable are relatively low which significantly limits the number of applications to which the systems may be applied. Coverage of a large area using infrared technology is particularly expensive, even more expensive than the radio communication equivalent. While point-to-point or line-of-sight infrared technology is cheaper than radio technology, it is unsuitable for most wireless LANs.
The main constraint on using any wireless LAN is interference. Infrared transmission particularly suffers from inter-symbol interference produced by multipath propagation effects. Achieving full coverage in an operating environment, while keeping within the limits of eye safety presents another problem to system designers, as does contending with interference produced by natural and man-made light sources that might be present. A further problem occurs when designing infrared receivers for such systems in that the receivers must provide the required sensitivity and bandwidth at minimal cost.
Achieving full room coverage while avoiding multipath propagation are conflicting requirements in a wireless infrared LAN and full room coverage is essential if reliable communication to and from any point within the room is needed. In order to achieve full room coverage, it is necessary to diffuse the transmitted infrared radiation. While diffuse transmission will reflect off walls and ceilings to fill the room the signal power reaching a given receiver is usually very small necessitating the use of very sensitive receivers. The problem of sensitivity is compounded by the necessity to detect a weak information bearing signal in strong interference. As a result of these problems there are few commercial infrared wireless LANs or associated systems available on the market.
Commercially available systems therefore tend to use elaborate structures and circuitry, which are expensive when compared with radio technologies. While there is a dearth of infrared technologies that support multi-user communication, there are numerous infrared technologies that support directed line-of-sight transmission. The most commonly available of these is the infrared serial port link based on the Infrared Data Association (IrDA) Standard. IrDA links can operate at data rates up to 4 Mb/s and are used in relatively inexpensive IrDA access points. However, IrDA links are only guaranteed to operate over a one meter range and are limited generally to one-to-one communication. IrDA is primarily intended as a replacement for a single wire-connection and is not intended for multi-user access. Notwithstanding this limitation, the popularity of IrDA clearly illustrates the enormous demand for reliable wireless technology. The best way to achieve multi-user links is to flood the operating environment with infrared light. While this enables multiple-users to connect to a network from anywhere in the operating environment, significant power is lost in such a diffuse environment thereby compromising the signal-to-noise ratio (SNR) at the receiver.